The picornaviruses are a family of small positive sense single stranded RNA viruses that cause a wide range of diseases at an annual cost well into the hundreds of million dollars. Members include acute hepatitis A virus, the heart disease-causing coxsackie B3 virus, rhinoviruses that cause more than half the occurrences of the common cold, and the paralyzing poliovirus. These viruses share a common life cycle where their RNA replication and viral assembly occurs in large membrane anchored replication complexes assembled on the surfaces of vesicles derived from the endoplasmic reticulum. The replication process is driven by a virally encoded RNA dependent RNA polymerase, the 3Dpol protein, that is responsible the synthesis of all viral RNA. Like all picornaviral proteins, the polymerase is generated by proteolytic cleavage of a single large viral polyprotein. There is mounting evidence in several picornaviruses that the polymerase and its immediate precursors are directly responsible for the assembly of these replication centers. The 3Dpol polymerase of poliovirus, the best studied of the picornaviruses, has been shown to assemble into large sheet structures along a protein-protein interface that was initially identified in a partial crystal structure of 3Dpol. We have solved the complete crystal structure of 3Dpol at 2.0 A resolution and discovered that the enzyme requires a free N-terminus to properly fold the active site, providing a molecular basis for the processing dependent activation of the polymerase. We are continuing our structural studies of the poliovirus proteins by further characterizing the conformational flexibility of 3Dpol and expanding into determining the role of protein-protein interfaces in the assembly of 3CDpro and other precursor proteins. The results will yield fundamental insights into the replication of this important group of pathogens.